Rotary percussion drill with a hard material bit

ABSTRACT

A rotary-percussion drill, includes a hard material bit ( 2, 2 ′) having at least one cutting edge ( 3, 3 ′), and a shaft having an end ( 7, 7 ′) with an axial base surface ( 6, 6 ′) provided with a chamfer ( 8, 8 ′) at least on its outer radial rim, the bottom surface ( 4, 4 ′) of the bit ( 3, 3 ′) which lies opposite the cutting edge ( 3, 3 ′), being secured to the base surface ( 6, 6 ′) of the shaft end ( 7, 7 ′) by brazing alloy in a material-locking manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rotary-percussion drill with a brazed hard material bit, in particular, to a rotary-percussion drill having a diameter from 4 mm to 45 mm for use with rotary-percussion hand-held power tools.

2. Description of the Prior Art

Usually, rotary-percussion drills of a type described above are provided, at the drill shaft end, with a chisel-shaped hard material plate that is brazed to the shaft end axially at its bottom and also over two opposite side attachment surfaces which are received in a receiving slot extending diametrically in the shaft end. Different heat expansion coefficients of the shaft end and the hard material bit in a direction normal to the axial direction, which are caused by large temperature differences which occur during butt-brazing and during percussion drilling, lead to high internal stresses, in particular in the brazing alloy that connects in a material-locking manner the bit with the shaft end. High internal stresses can cause failure of the drill. In principle, a small thickness of the brazing alloy layer increases the connection strength of the material-locking connection, however, it also increases internal stresses in the brazing alloy and the hard material bit.

German Publication DE 102 08 631 discloses a plate-shaped hard material bit having a bottom surface located opposite the cutting edges, and two attachment surfaces extending transverse to the bottom surface. Between the axial bottom surface and the side attachment surfaces, there are provided respectively, chamfers associated with the inner rims of the receiving slot in the shaft end. The two free surfaces that extend transverse to the two attachment surfaces and to the bottom surface of the bit, pass into the bottom surface without any chamfers.

German Publication DE 44 19 641 discloses a compact hard material bit that is butt-brazed with its bottom surface, which is located opposite the cutting edges, to the base surface of the drill shaft, without provision of any chamfers.

German Publication DE 44 42 266 discloses a cross-shaped hard material bit the bottom surface of which has a uniform, circumferentially extending chamfer having the same width.

German Publication DE 100 06 936 discloses a rotary-percussion drill which is optimized for drilling of reinforced concrete and has an X-shaped hard material bit formed of a sintered material. The bit has two diametrical side attachment surfaces and an axial bottom surface with which the bit is brazed in a corresponding X-shaped mortise and is butt-brazed to the axial base surface in a sector defined by two diametrical free surfaces.

An object of the invention is to provide a rotary-percussion drill for drilling of reinforced and non-reinforced concrete and having an improved service life.

SUMMARY OF THE INVENTION

This and other objects of the present invention which will become apparent hereinafter, are achieved by providing a rotary-percussion drill, including a hard material bit having at least one cutting edge and a bottom surface located opposite the at least one cutting edge, a shaft having an end with an axial base surface provided with a chamfer at least on its outer radial rim, and a brazing alloy for securing, in a material-locking manner, the bottom surface of the bit to the base surface of the shaft end.

The radial chamfer, which is provided on the outer radial rim of the base surface of the shaft end, extends at brazing with a brazing alloy over a wedge-shaped radial rim region. As it has been shown at a corresponding simulation according to Finite-Element-Method (FEM), as a result, the von-Mises reference stress and the hydrostatic stress condition, which can be considered with brazing alloy as failure criterium, have no extreme values at the rim where they can initiate fissure formation. In addition, the wedge-shaped radial rim region serves for receiving and for compensation of an excess of the brazing alloy which is hardly tolerated during mass production at brazing.

Advantageously, the axial thickness of the brazing alloy provided between the base surface of the shaft end and the bottom surface of the bit overall is smaller than 0.6 mm. However, the connecting strength of the material-locking connection is adequately high.

Advantageously, with a hard material bit having an X-shape with four radially extending blades, and the base surface having a corresponding X-shaped mortise in which the bit is brazed, with the bottom surface being butt-brazed to the base surface of the shaft end in a sector of two diametrical free surfaces, a radial chamfer is provided for each blade. Thereby, separate radial chamfers can be formed the same or be different.

Advantageously, in a radially middle region of the sector of two diametrical free surfaces, there are provided two additional radial chamfers between each two circumferentially adjacent blades. Thereby, a relatively large receiving capacity for the tolerated amount of the brazing alloy is provided, without the brazing alloy being squeezed radially outwardly, which improves removal of abrasively removed drillings, and, thereby, the drill feed.

The novel features of the present invention which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:

FIG. 1 a cross-sectional view of a rotary-percussion drill according to the present invention along line I-I in FIG. 2;

FIG. 2 a plan view of a shaft end of the drill shown in FIG. 1 without the hard material bit along line II-II in FIG. 1;

FIG. 3 a side view of another embodiment of a rotary percussion tool according to the present invention;

FIG. 4 a cross-sectional view of the drill shown in FIG. 3 along line IV-IV in FIG. 3;

FIG. 5 a plan view of the shaft end of the drill shown in FIGS. 3-4 without the hard material bit in the direction of arrow V in FIG. 4;

FIG. 6 a longitudinal cross-sectional view along line VI-VI in FIG. 5;

FIG. 7 a longitudinal cross-sectional view along line VII-VII in FIG. 5; and

FIG. 8 a longitudinal cross-sectional view along line VIII-VIII in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A rotary percussion drill 1 according to the present invention, which is shown in FIGS. 1-2, has a plate-shaped hard material bit 2 with a diametrically extending cutting edge 3 and a bottom surface 4 located opposite the cutting edge 3. The bottom surface 4 is secured with a brazing alloy 5 in a material-locking manner to a base surface 6 of the drill shaft end 7 in an adapted slot-shaped mortise 9. The base surface 6 is provided on diametrically opposite side of an outer radial rim with a radial chamfer 8. The axial thickness D of the layer of the brazing alloy between the base surface 6 and the bottom surface 4 of the bit 2 is overall smaller than 0.6 mm.

A rotary-percussion drill 1′, which is shown in FIGS. 3-8, has an X-shaped hard material bit 2′ with four radially extending blades 11 with respective cutting edges 3′. The hard material bit 2′ is secured with brazing alloy 5′ in a correspondingly adapted X-shaped mortise 9′ and is butt-brazed with its bottom surface 4′ to an axial base surface 6′ of the drill shaft end 7′ in the sector defined by two diametrical free surfaces 10. For each blade 11, there is provided a radial chamfer 8′. In the sector of the two diametrical free surfaces 10, in the radially middle region, between two, circumferentially adjacent blades 11, there are formed two additional radial chamfers 12.

FIG. 5, as discussed above, shows a plan view of the drill shaft end. FIGS. 6-8 show the cross-sectional shape of the brazing alloy layers between the bottom surface 4′ of the bit 2′ and the base surface 6′ of the shaft end.

Though the present invention was shown and described with references to the preferred embodiments, such are merely illustrative of the present invention and are not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims. 

1. A rotary-percussion drill, comprising a hard material bit (2, 2′) having at least one cutting edge (3, 3′) and a bottom surface (4, 4′) located opposite the at least one cutting edge (3, 3′); a shaft having an end (7, 7′) with an axial base surface (6, 6′) provided with a chamfer (8, 8′) at least on an outer radial rim thereof; and brazing alloy means (5, 5′) for securing, in a material-locking manner, the bottom surface (4, 4′) of the bit (3, 3′) to the base surface (6, 6′) of the shaft end (7, 7′).
 2. A rotary-percussion drill according to claim 1, wherein an axial thickness (D) of a layer of the brazing alloy means (5, 5′) provided between the base surface (6, 6′) of the shaft end (7, 7′) and the bottom surface (4, 4′) of the bit (2, 2′) overall is smaller than 0.6 mm.
 3. A rotary-percussion drill according to claim 1, wherein the hard material bit (2′) has an X-shape with four radially extending blades (11), and the base surface (6′) has a corresponding X-shaped mortise (9′) in which the bit (2′) is brazed, with the bottom surface (4′) being butt-brazed to the base surface (6′) of the shaft end (7′) in a sector of two diametrical free surfaces (10), and wherein the base surface (6′) is provided with a radial chamfer (8′) for each blade (11).
 4. A rotary-percussion drill according to claim 3, wherein in a radially middle region of the sector of two diametrical free surfaces (10), there are provided two additional radial chamfers (12) between each two circumferentially adjacent blades (11). 